The sale fares also include longer routes, with the price of flights loosely tied to distance. Flights begin at $49 each way on Southwest’s shortest routes and increase to $79, $99 or $129 each way for longer flights.

The sale launched Tuesday morning and is good for travel from Aug. 22 through Dec. 13. Flights on Fridays and Sundays are excluded from the sale as are certain dates around the Labor Day and Thanksgiving holidays. Schedules for Southwest’s Florida and Nevada flights have additional day-of-week restrictions.

The sale fares apply specifically to nonstop options, though many connecting itineraries may also show lower-than-usual fares. Seats sold at the sale prices are capacity controlled, meaning the cheapest seats will likely sell out on individual flights. Southwest’s flights from Albany, N.Y., and Manchester, N.H., are excluded from the sale altogether.

Many of Southwest’s international routes are also included in the sale, though those routes come with more day-of-travel restrictions. Fares on those routes range from $59 to about $250 each way, but are good only for flights on Tuesdays and Wednesdays during the sale period.

Regardless of the details, bargain seekers will have to act quickly to snag the fares. The sale ends on Thursday (June 8) at 11:59 p.m. local time in city of the departing flight. (Full sale details)

However, those used to Southwest’s previous sale fares should be warned that the availability of Southwest’s cheapest fares appears to be somewhat more limited than in past sales. The $49 fares do appear on most days on the advertised routes, but they do not always appear on every flight, according to a quick check of the carrier’s website early Tuesday morning.

Using the airline’s Boston-Baltimore route as an example, the $49 one-way fares were available on just three of the airline’s 10 non-stop flights on Nov. 1 (a Wednesday). Fares on five of the day’s other flights were going for a still-low $69 while two others were selling for $150 one way. A day later, though, nine of Southwest's 10 daily flights from Boston to Baltimore were on sale for $49 as of early Tuesday morning.

Other routes showed the broad availability that have helped make Southwest’s twice-a-year sales so noteworthy. A spot-check of Southwest’s San Francisco-to-Los Angeles for the same day (Nov. 1) showed all 10 of the day’s flights were available for $49 as of early Tuesday morning. The same was true of all 10 flights in the opposite direction for Nov. 7 (a Tuesday).

Fares on some routes dropped even lower than the advertised fares. Flights between Spokane, Wash., and Boise – for example – were selling for as little as $92 round-trip early Tuesday morning. The same was true for a handful of other routes, including a limited number of $90 round-trip fares for Southwest’s flights between Chicago Midway and Minneapolis/St. Paul.

he takeaway: While the sale fares are widely available through the sale window, customers with the greatest schedule flexibility will have the best luck securing the lowest advertised fares. Still, fliers should remember that the cheapest seats will likely begin to sell out as the sale goes on.

The broad fare sale has become a staple for Southwest. It has rolled out similar three-day sales each June and October for the past several years. One sale from June 2015 proved so popular that it crashed Southwest’s website, prompting the carrier to extend that particular sale by an additional 24 hours.

In previous iterations of the sale earlier this decade, Southwest pegged fares to mileage thresholds. For example, Southwest 's sales priced flights of 500 miles or less at $49 each way and increased from there. Flights of 501 to 1,000 miles cost $99 each way and flights of 1,001 to 1,500 miles cost $129 each way. Flights of more than 1,500 miles went for $149 each way.

While Southwest has ended the precise mileage component of its big sales, its latest versions do closely mimic the previous distance-based sales. The carrier's four advertised sale-fare tiers — $49, $79, $129 and $149 each way — are similar to the prices offered on the distance-based sales of years past.

Whatever the details, travelers can snag advertised round-trip fares as low as $98 on short routes. Even on the airline’s longest cross-country routes, fares are as low as $258.